Linux: Fix a major regression in threads_posix.c

Linux: Fix a major regression in threads_posix.c

* On some Linux platforms, libusbx compilation breaks with:
  error: implicit declaration of function ‘pthread_mutexattr_settype’
* This regression, introduced in 463dda06db5da5de0eab32820c7af60605625afe,
  is due to pthread.h needing __USE_UNIX98, which is tied to _XOPEN_SOURCE
  or _GNU_SOURCE being correctly defined, and which the inclusion of
  <unistd.h> before the _XOPEN_SOURCE override modified
* As _GNU_LINUX ensures the definition of __USE_UNIX98 and we require it
  for syscalls, we now only define _GNU_SOURCE for Linux.